Evolution of the Application of Composite
Materials to Helicopters
R.L. Foye
J o h n L.. Shipley
Aerospace Engineer
U.S. Army Research and Technology Laboratories
(AVRADCOM),MoffettField, Calif.
Deputy Director, Applied Technology Laboratory
U.S. Army Research and Technology Laboratories
(AVRADCOM),Fort Eustis, Va.
This paper contains a brief chronology of the major US developments in the application of composite
materials to helicopters. The major events of each decade from 1940 are described. Some referencesare made to
significant accomplishments in helicopter structures, materials development and the general application of
composites in order to establish a framework for the subject matter. Comments are also made on future trends
and new applications.
Background
T
HIS paper contains a brief chronology of the major US
developments in the application of composite materials to
helicopters (omitting turbine engine applications). The paper
emohasizes militarv rather than civil R&D because the bulk of
the progress has taken place in the former sector.
In a review of this tvoe. it is difficult to isolate the subiect
matter from broader ddvilopments in the areas of helicopter
technology, structures, materials, and general aerospace application of composites. Therefore, some references are made
to significant accomplishments in these broader areas to place
the subject matter in the proper perspective. Some references
to foreign R&D work are also included.
Beginning with the 1940s. the major events of each decade
are described. Prior to 1940, there were no practical composite materials (from an aerospace viewpoint) and no practical
helicopters. However, some interesting earlier parallels or
coincihencec in the two technologies have occurred. As early
as the Renaissance (circa 1500), both the concept of vertical
flight by means of powered airscrews and the concept of
reinforcement of a weaker material through the introduction
of stronger and stiffer fibers were known and documented in
the works of da Vinci.' Within this century, 1909 was a
milestone year for both technologies with the first successful
helicopter flight and the development of the first plastic suitable for laminating ( p h e n ~ l i c ) .1922
~ was also doubly significant in that a US patent was awarded to Robert Kemp of the
Westinghouse Corporation for the concept of an all composite airframe3 while the first US Army helicopter contract of
$19,800 was awarded to George de Bothezat. De Bothezat's
helicopter reached a height of several feet but the concepts
contained in the Kemp patent languished for almost two
decades.
The pre-1940 period can best be summarized as a search for
a practical starting point for the development of both helicopters and composite materials. The eariier helicopters contninprl I h~urilrlerinrrvnrietv nf eoncentr. In comnnsites. the
only reinforcing fibers available were the natural fibers and
some low modulus synthetics. Phenolic molding was a high
pressure process not well suited to the fabrication of complex
shapes. In brief, composites and helicopters had yet to
establish a firm basis of their own let alone find a common
ground. However, within a short period, major deveiopments
in both areas would mark the beginnings of their modern eras
and the application of composites to helicopters would soon
follow.
The Period 1940 Through 1949
The event that marked the beginning of the modern era of
composites was the commerciaiavail~bilityof fiberglass in
1940. Epoxy resins had been patented and produced in Switzerland in the late 1930s, but within this country polyester resin
was the nrincinal matrix material throuahout the 1940s and
well into'the 1950s. The combination of liberglass and polyester comoriscd the first aerospace-grade
com~ositematerial. In
.
.
World w a r I1 this material saw application to radomes, fairings and rocket launch tubes.
1940 was also distinguished by the free flight of what is generally considered to be the first practical helicopter, the
Vought-Sikorsky VS-300. Its fuselage and landing gear structure was a metal truss partially covered with fabric. The main
rotors were steel tubes with a wood and fabric aerodynamic
covering. Two years later the Sikorsky R-4 became the first
helicopter t o enter US military service. The R-4 was primarily
truss/fabric construction with some removable sheet metal
covering in the forward fuselage. The rotor was again a steel
tube with wood and fabric fairings. The Sikorsky R-5A which
followed in 1943 had a similar blade concept but the fuselage
center section was covered with a composite of wood and
plastic. The R-5A forward fuselage used all metal construction. The tail section was wood monocoque. The
Sikorsky R-6A, which was also introduced in 1943, had the R4 tvne rotor blade but the fuselaee had naoer/olastic com-
JOURNAL OF THE AMERICAN HELICOPTER SOCIETY
posite molded cowlings and a fiberglass reinforced plastic
(FRP)
. , floor cover in^.
- The rest of the fuselage
. was metal
monocoque.
Through World War I1 almost two dozen different US helicopters were developed and flown. The all metal covered Bell
Model 42 which flew in 1945 indicates the extent that
aluminum sheet metal came to dominate helicopter fuselage
construction during this period.
In the rotor blade area, the first all metal riveted steel rotor
blade flew on the coaxial Hiller XH-44 in 1944. By the end of
World War 11, an all metal steel and aluminum bonded main
rotor flew on a Sikorsky S-51.
During the wartime period several experimental composite
components of fixed-wing aircraft were built and tested.'
These included a Vickers Spitfire fuselage made from hemp/
phenolic; a Vultee BT-I5 aft fuselage that was flown in 1944,
and an outer wing of an AT-6 which was static tested at
Wright Patterson Air Force Base in 1945 but did not fly until
almost 10 years later.
In the period immediately following the war, epoxies became commercially available in the US and the automated filament winding process was developed. At least two dozen additional US helicopter designs flew before the close of the
1940s. The Bell Model 47 was the first to be commercially certified in 1946. In 1947 Cornell Aero Lab built the first rotor
blades which used some FRP construction. These blades had
wood spars with FRP skins and flew on a Sikorsky R-5. This
was a year before the first production all metal rotor blades
flew on the Sikorsky S-52. Plastic fuel tanks appeared as early
as 1948. Magnesium skins were in production as early as 1949.
By the end of the 1940% all metal semi-monocoque fuselages and hybrid steel/wood rotor blades were accepted practice in helicopter design. The future trend appeared to be in
the direction of all metal blades. Composites had made few
inroads except in R&D and prototype development. Throughout the 1940s both helicopters and composite materials had
grown in status from somewhat vague experimental concepts
to viable realities. However, the extent of their future potential remained uncertain. It would take another war and the
realization of the shortcomings of metal construction to
merge the two technologies and firmly establish their present
roles.
materials (Fig. I). An experimental FRP landing gear for the
YH-32 was also built and tested but was not put into production. In 1953 Glenview Metal Products built a GMP-2 Flyride
helicopter with a main rotor blade of laminated spruce forward of the 30% chord and FRP skins on a balsa wood core
aft of the 30% chord. In the same year Jacobs Aircraft Engine
Company built and flew a helicopter (Model 104)with a fuselage of welded steel tubes enclosed in a molded FRP skin. In
1954 the Air Force contracted with Piasecki Aircraft to design
an FRP center fuselage section for their H-21 helicopter.
Several large composite curved segments of the center fuselage were built and static tested by Boeing Vertol (Piasecki's
successor) in 1957. They were made primarily of cocured aluminum honeycomb sandwich construction with composite
face sheets and some partial frames (Fig. 2). Boeing Vertol's
conclusions were that FRP primary structure could be made
without a weight penalty and the potential existed for lower
structural costs due to simplified fabrication methods. In 1956
Prewitt Aircraft Company built three sets of research rotors
for the Piasecki HUP-2 aircraft. One was made of stainless
steel, one of titanium, and one of FRP. The details of this design are not known but it may have been the first all composite blade to fly on a helicopter. Parsons also built experimental blades for the H-21 from stainless steel and FRP in the
same period. Bell built an FRP cabin enclosure skin for its
Model 47 in 1956.
Two of the most significant events of the late 1950s went almost unnoticed at the time. They were the appearance of the
first graphite fibers in 1958 and the first boron fibers in 1959.
These fibers were expensive lab curiosities whose mechanical
properties underwent great improvements in the early 1960s.
However, their appearance eventually led to the flood of activity in aerospace structural applications of composites that
began in the mid-60s and continues to this day.
In summary, the period of 1950 through 1959 was marked
by general acceptance of the helicopter as an agricultural tool,
an emergency civil vehicle, and a military transporter; having
proved its worth in the Korean War. Existing rotor blade design practice had changed from wood or hybrid wood/metal
The Period 1950 Through 1959
The 1950s were notable for helicopter engine improvements
rather than structural ones. The development of the turbine
engine was uniquely accountable for the present role of the
helicopter in civil and military aviation. The first US turbine
powered helicopter, a Kaman K-225, flew in 1951. The first
production turbine helicopter, the Bell UH-I, flew in 1956.
The most significant production helicopter structural change
in the 1950s was the continuing trend toward metal rotor
blades as a means of avoiding the moisture absorption and
blade tracking problems associated with wood blades.
In the fixed-wing area, several all FRP light aircraft were
built and tested such as the Piper Papoose, Mississippi State
Marvelette, and Taylorcraft Model 20. They were airworthy
but the economics of ~roduclionruled out thcir development
beyond the prototype stage. However, European production
of high performance all FRP gliders did begin in 1958.
Through the decade FRP composites became a standard material of construction for fairings, ducting and other secondary
fixed wing airframe structure, particularly for complex
curved parts which were difficult to form in metal. FRP also
made its first volume inroads in the automotive industry with
the Woodhill Wildfire (1950), Kaiser Darrin (1951), and
Chevrolet Corvette (1953) roadster bodies.
The decade was not without some notable accomplishments
in the composite helicopter area. One of the most remarkable
was the Hiller-HJ2 (Hornet). The fuselage primary structure
was a metal truss entirely enclosed in an FRP skin. An Army
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Fig. 1 Hiller YH-32
OCTOBER 1981
to all metal. Fuselage technology had changed only to the extent that the all metal fuselage was more firmly established
than ever. FRP composites had become an acceptable candidate for application to secondary structure only. Even the
R&D efforts of the time had not explored the application of
composites to primary helicopter flight structure, beyond a
few isolated tests.
The Period 1960 Through 1969
The 1960s saw a substantial increase in the level of activity
of composite applications within the general aerospace community. This was stimulated by the first commercial availability of the advanced composites, boron/epoxy, followed by
graphite/epoxy a few years later. The specific mechanical
properties of these composites were sufficient to assure significant weight savings over the light alloys in almost all applications. This had seldom been the case with FRP. On this basis,
the 1963 Air Force Project Forecast recommended that advanced composite primary structure become a major R&D
thrust. This in turn led to the formation in 1965 of the Advanced Composites Division of the Air Force Materials Lab
with the funds to pursue that goal.
Much of the early emphasis was on fixed-wing lifting surfaces which were generally stiffness critical and showed the
most promise for advanced composite application. However,
rotor blades were the beneficiary of some of the effort. Despite this work, all metal rotor blades continued to dominate
helicopter technology of the 1960s. Change was manifest
mostly in the increased use of metal honeycomb sandwich
construction and the reduction in the numbers of mechanical
fasteners in blades, as a result of fatigue problems associated
with fasteners. There was a corresponding increase in emphasis on adhesive bonding as a means of reducing or eliminating
fasteners.
The early 1960s saw the introduction of several composite
rotor blade designs. Among these were the Kaman H43-B
blade of 1960 which had a wood core and FRP skins on the
afterbodies. The Boeing Vertol CH-47 blade of 1961 consisted
of a steel D spar with aluminum ribs and FRP skins. In 1962,
Kaman flew an all FRP blade on the HH-43B. The cost of this
blade, with its on-for-one metal part replacement philosophy,
was too high to compete with production metal designs.
In the composite fuselage area, an important development
program of the 1960s was the Sikorsky in-house program that
led to the cocured FRP cockpit for the H-53 (Fig. 3). This program was particularly significant because it succeeded in substantially reducing the cost of this complex assembly over the
EVOLUTION O F COMPOSITES
7
baseline metal design. It did this by reducing the part and
fastener counts and by designing and tooling to minimize
composite fabrication cost.
With the advent of advanced composites, their applicability
to helicopter structures was investigated in a 1966 Bell study.4
This work showed, rightfully at the time, that there were no
major benefits in applying composites to helicopters except in
the rotor blade area. Only the subsequent appearance and
gradual reduction in the cost of graphite fibers and the appearance of Kevlar fibers several years later would change the
basis of this conclusion. As a result, emphasis shifted for the
remainder of the decade to the rotor blade area. By 1968 Sikorsky had flown a boron composite tail rotor on an S-61
(Fig. 4) and the same year marked the beginning of the all
boron composite Advanced Geometry Blade program for the
Boeing Vertol CH-47 main rotor (Fig. 5).' An all FRP version
of this blade was built soon afterward.
By the end of the decade, dozens of aerospace components
had been built out of advanced composites, but these materials made no substantial production inroads until the 1970s.
In the meantime, NASA began working on composite reinforced metal concepts for fuselages and lifting surfaces. This
would soon lead to helicopter applications.
In summary, the decade of the 1960s, mainly through the
experiences gained in Southeast Asia, saw the utilization of
the helicopter expanded from that of a transport vehicle to
Fig. 4
Boron/Epoxy Tail Rotoron S-61.
JOURNAL OF THE AMERICAN HELICOPTER SOCIETY
Fig. 6 Composite appliestionsan the UH-60A,
Fig. 7 Composite on theYAHd4.
a
(Fig. 6) and Hughes AAH (Fig. 7) had between 20 and 25% of
their wetted area in composites. The Sikorsky S-76 (Fig. 8)
had over 30%. In fixed-wing aircraft, the McDonnell-Douglas
AV-8B has over 75% of its surface covered with composites.
This included considerable primary structure. Some research
vehicles such as the Air Force HIMAT and the NASA AD-1
oblique-wing aircraft were essentially all composite.
Another notable feature of the 1970s was the flight service
evaluation programs to investigate the reliability, repair,
maintenance, moisture pickup, and possible long-term degradation of composite components. These programs also attempted to correlate long-term behavior of composite flight
components with various real and accelerated time lab and
outdoor exposure tests. These tests were designed to simulate
the effects on materials of actual military and civil fleet usage
in both moderate and extreme operating environments.
I n the materials area, the 1970s were marked by the appearance of Kevlar reinforcine fibers and the develooment of
more easily processable high temperature resistant polyimide
matrix materials. Comoosite materials also survived a host of
crises, real and imagined. These ranged from high velocity
particle erosion resistance problems to low velocity impact
damage susceptibility, long-term moisture absorption (with
possible mechanical property degradation) and the fear of
wide-spread civil property damage from airborne graphite
fibers originating at an aircraft crash site.
Helicopter structures, in general, saw a significant increase
in the use of nonmetallic sandwich core material in place of
aluminum core, which was prone to corrosion. The applications of titanium increased, particularly in the areas of main
rotor blade spars and hubs, which led to some regrets as the
raw material and its reserve processing capacity diminished.
The appearance of elastomeric bearings in main and tail rotor
hubs and controls was also a notable development. However,
the most sienificant helicooter structural event of the decade
was the onrush olcomposite R&D and the application of this
material to primary flight structure. The decade began with
thc emphasis on rotor hladcs but fuselage structure was soon
cauaht ur, in the tide which influenced the dcsign of virtually
all the major structural components.
In the main rotor blade area, 1970 and 1971 saw the Boeing
Vertol Advanced Geometry Blades fly on the CH-47. Shortly
after this, Messerschmidt-Boelkow-Blohm (MBB) of Germany put the first all FRP main rotor blade into production
on the BO-105. In the US Army Heavy Lift Helicopter
prototype programs an all composite main rotor blade was
developed and lab tested (Fig. 9) but never flight tested due to
Congressional cancellation of the program in 1975.
In the second half of the decade, a number of all composite
main rotor blades were developed. In 1975, an FRP multitubular filament wound blade (Fig. 10) was developed by
Hughes and Fiber Science9 for the AH-IG and Sikorsky built
an all composite blade for the H-53. Later, as a part of the
Army Product Improvement Program for the AH-IQ, an all
composite main rotor blade was flown in 1977 (Fig. 11). It
was built bv Kaman and Hercules.l0 Shortly after that, Boeing Vertol built and flew all composite blades for both the
CH-46" and the CH-47D (Fig. 12). "
In another programI3 a bearingless main rotor concept was
applied to the BO-I05 by Boeing Vertol. In this concept the inboard end of the blade is designed to deform readily in torsion
but remains stiff in bending thereby eliminating the need for a
torsional hinge. This rotor first flew in 1978 (Fig. 13). Bell
had also developed and flown FRP blades for their Model 214
and 206L helicopters in the late 1970'~.".~'A Kevlar/epoxy
main rotor blade for the Hughes AAH (Fig. 14) (the first all
Kevlar/epoxy main rotor) had been built and flight tested on
an AAH prototype. Many other composite rotor blades were
desiened in this time oeriod but did not reach flight test stages
for various reasons.
In the tail rotor area. the R&D that led to the composite
bearingless flex-beam tail rotor was well underway by 1970
-
KEVLAR /EPOXY
FIBERGLASS/EPOXY
GRAPHITE/EPOXY
Fig. 8 Composite Applications on the S-76.
that of a military combat vehicle. The decade also saw the
widespread use of composites confined to secondary structure. However, some small production inroads were made in
primary cockpit structure and the all composite rotor blade
concept had taken a large step forward with the Advanced
Geometry Blade programs. The future of helicopter rotor
blades was now clearly pointed in the direction of all composite con~truction.~The shortcomings of metal blades
(corrosion, fatigue, and rapid crack propagation) had been
recognized and composites offered a solution to these problems. Composite inroads into primary fuselage structure had
gained some small ground but the bulk of that R&D would
have to wait for the next decade.
The Period 1970 Through 1979
The most recent decade of aerosoace aoolications
of com..
lposites can be cl~aracteriredas one of realization. Most of the
;airframes that were designed in the 1970s had significant comnosite auolications. TG Sikorsky UH-60 BLACKHAWK
~
~
~-
WEDGE
.NOSE
TITANIUM
FIBERGLASS
BALANCE LEADING EDGE
" D SPAR
WEIGHT
Fig. 9 HLH composite main rotor.
Fig.
10
Fig. 1
Filamenl wound blade for
AH-IG
Improved main rotor blade for
AH-I.
(Fig. 15). By mid-decade, this design innovation had become
the industry standard (Fig. 16). The UH-61, and S-76 used
this tail rotor concept and a bearingless tail rotor is now under
development for the AAH.
Composite tail rotor drive shafts have been under development throughout most of the 1970s. Bell and Sikorsky R&D
supported earlier graphite drive shaft work (Fig. 17).16 The
Army Labs have pursued various composite hybrid and imoact resistant resin conceots to reduce the vulnerabilitv of the
shafts to accidental maintenance damage. l7
Among the other helicopter dynamic components that have
been developed in composites, a boron/epoxy stiffened aluminum swashplate was built by Boeing Vertol in 1973 for the
Heavy Lift H e l i c ~ p t e rand
' ~ a main transmission housing for
the UH-I was built by Whittaker using filament wound
graphite/epoxy (Fig. 18).19 Heat dissipation problems delayed any follow-up to this transmission effort. A few years
later, the concept was revived by Boeing Vertol using a
vacuum infiltrated metal matrix process that is still under
development.
Composite main rotor hubs became a reality in the 1970s.
Kaman built a half-scale composite hub for the CH-54 in 1975
(Fig. 19) that showed great promise of cost and weight savings.l0 R&D funding shortages have needlessly delayed this
application to US helicopters. In the meantime, Aerospatiale
has put three and four bladed composite Starflex hubs into
production on the ASJSO, the SA-365, and the Coast Guard
SRR helicopters.
Now consider some of the recent applications of composites
to the static components of the helicopter airframe. In the
10
R.L. FOYE
JOURNAL OF T H E AMERICAN HELICOPTER SOCIETY
Fig. 12 Composite blades for CH.47D.
Fig. 15 Bearingless Tail Rotor on Bcll Model 206.
Fig. 13 Bosringlcss main rotor for BO-105.
Fig. 16 Bearingless tail rolor a n Sikorsky UH-60.
Fig. 14 Kevlarmain rotorforYAH-64,
early 1970s various US and European R&D programs explored the application of composites to tail cone structure.
This was the first venture into helicopter primary fuselage
structure, excepting the FRP cockpit development work of the
1960s. As a part of this effort Bell looked at filament winding
as a means of fabricating the AH-lG tail cone." Two of these
graphite/epoxy sandwich shell structures were built and
ground tested (Fig. 20). Westland investigated FRP and
graphite/epoxy tail cones for the Wasp helicopter and MBB
tested composite tail cones for the BO-105.22 About the same
time, Sikorsky built and flew a boron reinforced metal tail
cone on the last production CH-54B (Fig. 21).13 The structure
eventually entered into a composite flight service evaluation
program in 1972 accumulating over a thousand flight hours of
trouble-free service before it was severely damaged on the
ground and scrapped. This vehicle also had a unique
boron/epoxy tail skid design (Fig. 22).
Fig. 17 Composite tail rotor drive shaft section.
The flurry of composite tail cone activity continued
through the decade with a second filament wound AH-1G tail
cone design that flew in 1976 IFia. 23)." a com~ositetail cone
for rhr ~erosparinle~ a u p h i n ~ x - 3 6 5 ; and
" several filament
wound ballistic tolerant rail cone ronccpts huilt and teslcd by
the Army lab^.^'
EVOLUTION OF COMPOSITES
OCTOBER 1981
Fig. 18 Filament wound transmission housing.
Fig. 21
Boron stiffened tail come for
CH-54,
Fig. 19 CH-54composlte plate main rotor hub.
Fig. 22
Boron/epoxy tail skid an CH.54.
Fig. 23
Fia. 20
G r a ~ h i t e / e ~ o xlail
s cone far AH-I
Fie. 24
AH-l eomposilc lsil cone.
Composite flight control eompanenls.
12
JOURNAL O F T H E AMERICAN HELICOPTER SOCIETY
R.L. FOYE
Fig. 25 Kevlar air deflectors on CH-53.
PANEL M A T E R I L XEYLAR IO/POLYSULPHONE
/
li,
EECTiON A-A
\
>~, ~,
<
Fig. 27 G~aphitesliffened Kevlar skin panel,
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SECTION B-B
Fig. 26 Thermoplastic englne access door for CH-47.
Composites were also applied to helicopter flight controls
by the Army Labs and Rockwell International (Fig. 24).26
Several ballistic tolerant concepts for mechanical control
elements were desianed and tested. These were not used due to
the cost differential between the composite hardware and the
simole metal tubes and castings
. they
~-replaced.
The 1970s also signaled the spread of Kevlar/epoxy secondary structure as a lightweight replacement of FRP material in
minimum gage cowlings, fairings and other secondary structure. One of the first such components was a set of air
deflectors (called Beagle Ears) on the aft fuselage of a Sikorsky CH-53D (Fig. 25). The Kevlar/epoxy aft inlet fairing ;or
the Hughes OH-627was another example. The Sikorsky S-76
used Kevlar/epoxy secondary structure extensively (Fig. 8).
More recently, a Boeing CH-47 engine access door was made
from reinforced thermonlastic material (Fig.
- 26)
. and the large
external fuel tank, for ;he Boeing 234, a commercial version
of the CH-47. have been built from a Kcvlar/graphite hybrid
Fig. 28 Composite cabln roof segment.
---. ----- .
.r
In the com~ositefuselage cabin area there were a number of
important de;elopments in the 1970s. Early in the decade, as a
result of the Sikorsky FRP composite cockpit work and some
fixed-wing composite studies, it became evident that cocured
or "one-shot" assemblies were the best approach to fabricating cost effective composite structures. This led to a Sikorsky
CH-53D study that focused on low cost fabrication of composite skinstringer-frame construction which operated in the
post-buckling load range.2s Previously, sandwich construction and non-buckling stiffened skins were considered the
only composite fuselage design options available. This pro.h,...,~rl t h a t nnrt.hllrk~p,i ~ t i f f ~ ~ ..kin.
-n
V ~ ~
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OCTOBER 1981
EVOLUTION OF COMPOSITES
Fig. 30 Composite aft Section of OH-58.
Fig. 31 Bell 206L Helicopter camposile components.
13
I4
R.L. FOYE
JOURNAL OF THE AMERICAN HELICOPTER SOCIETY
Fig. 33 Double crescent shaped landing gear.
posite fuselage construction in the near future. These studies
had considerable influence over Army R&D planning.
In 1979, Sikorsky fabricated and successfully static tested
an all graphite cabin roof structure representative of the UH60, which contained the transmission attach fittings, longitudinal beams, frame segments, and some shear-carrying roof
skin (Fig. 28)." This is considered one of the most difficult
areas of composite design and fabrication. Sikorsky is also
building a composite fuselage transition section of the UH-60
BLACK HAWK from Kevlar and graphite/epoxy for the
Army (Fig. 29). It will be tested in the early 1980s.
The composite flight service evaluation programs for helicopters, which began with the CH-54B boron stiffened tail
cone, were expanded to include various composite materials
on the aft fuselage of the Bell OH-58 (Fig. 30). These components were made by Hughes and Fiber Science. l2Bell is evaluating various composites in the secondary and primary structure of a portion of the 206L commercial fleet (Fig. 3 V 2
In the landing gear area Hughes and the Army Labs have
developed filament wound landing gear components for the
AAH (Fig. 32)" and have done some concept evaluations of
composite crash energy absorbing landing gears (Fig. 33). Bell
has also tested graphite/epoxy crash energy absorbing seat
struts. l4
Composite helicopter development in the decade of the
1970s can best be summed up as one of remarkable progress
across a broad front. The highlights were the proliferation of
all composite main rotor blades, the acceptance of bearingless
composite tail rotors, Kevlar/epoxy replacing FRP in secondary structures, and R&D progress toward the adoption of
composites in primary helicopter fuselage structure.
and the Kevlar/epoxy had potential as a helicopter skin material (Fig. 27)
Both Boeing Vertol and Sikorsky investigated all composite
fuselage structure for an Army Medium Utility Transport
(MUT) helicopter in 1976.29.30Both studies concluded that
rivnificnnt weieht and cost savings were oossible with all com-
Predictions for the 1980s
The future application of composites to all parts of the helicopter fuselage structure now seems assured. The Army has
recently funded multiple Advanced Composite Airframe Programs (ACAP) with five major US helicopter airframe fabricators for the preliminary design of an all composite helicopter fuselage structure that will be built and flown by the mid-
EVOLUTION OF COMPOSITES
OCTOBER 1981
1980s. " Progress has also been made toward thc initiation of
an Armv/NASA Inleerated Tcchnoloav Rotor/Fliaht
Research R o t o r (ITRIFRR) project whichwill probably a s similate the comnosite rotor. comoosite h u b a n d bearingless
.
main rotor concepts into a single composite rotor system
sometime in the mid-1980s. With the c o m ~ l e t i o nof these nrograms, the all composite helicopter will' he a reality in- the
R&D sense. History has shown that production applications
will not h e far behind.
gaps in the a n ~ l i c a t i o nof comT h e ~ r i m a r vremaining...
positesto helifopters a r e the completion &he R&D work o n
comnosite transmission cases, landing gear a n d drive shafting: and ultimately, the phasing o f t h i b R & D into manufacturing technology and production. This will include improved
NDI a n d repair methods, less labor a n d energy intcnsivc manulacturing, a n d more comprehcn.;ivr flight service results. We
have already seen an increa.;cd emphasis o n manufacturing
technology t o assure a producible, good quality product a t a n
affordable cost.
None o f this will represent the last word i n the technology.
Large improvements remain t o b e made in the more efficient
application of these materials i n t h e content of current design
conceds.. a n d later. t h e develnment o f new design
- concents
better suited t o these materials a n d requirements. Also, the
materials a n d requirements a r e changing. There will he newer
VTOL concepts a n d newer fibers, matrices a n d forms o f
reinforcement. Tougher reauirements i n crash-worthiness,
maintainability and decreaseh vulnerability seem likely. However. the nrecise direction a n d magnitude o f emnhasis for this
or i n y L t u r e R&D must always remain uniertain by its
nature.
.
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